Mucous membranes of the gastrointestinal tract are constantly exposed not only to antigens derived from food and enterobacterial flora, but also to various antigens existing in the outside world that are harmful to the living body, such as pathogenic microorganisms. Therefore, the gastrointestinal mucous membranes exhibit a cytotoxic activity in order to compete against such antigens harmful to the living body. While maintaining the ability to secrete antibodies to neutralize toxins, these mucous membranes also have the unique immune mechanism of suppressing excessive immune reactions against antigens such as food and enterobacterial flora (this mechanism is called gastrointestinal mucosal immunity or intestinal immunity). Specifically, normal mucosal immunity is established on the balance between positive immune responses against pathogens and negative immune responses against non-pathogenic antigens. When this balance of immunological homeostatic maintenance collapses, inflammation, allergies, and infections occur, triggering the onset of intestinal diseases generally termed Inflammatory Bowel Diseases (IBD) and alimentary allergies.
The most representative inflammatory bowel diseases are Crohn's disease (CD) and colitis (especially Ulcerative Colitis (UC)). Both are diseases in which the pathogen cannot be specified and chronic and recurrent attacks of abdominal pain and diarrhea occur, causing significant, long-term obstacles to the daily life of child and juvenile patients. Furthermore, since colitis (especially ulcerative colitis) may become a causative for colon cancer, there is an urgent need for elucidating the pathogenesis of colitis and developing effective therapeutic methods.
Although various possibilities such as genetic and environmental factors have been discussed concerning the mechanism of onset for inflammatory bowel diseases, recent studies indicate the strong possibility that abnormal immunity of the intestinal tract (gastrointestinal mucosal immunity) may be the cause. More specifically, an inflammation or allergy occurs in the intestinal mucous membranes due to the induction of an excessive immune response that occurs for some reason against antigens in the intestine that are normally non-pathogenic and have a low immunogenicity, resulting in the onset of an inflammatory bowel disease.
Furthermore, abnormal immunity against foreign pathogens, antigens derived from food, or autoantigens has been suggested to be deeply involved in such inflammations and allergies of the intestine. Furthermore, recent studies have suggested the possibility that abnormal immune responses towards certain indigenous bacteria manifest as chronic inflammatory reactions.
This mechanism of onset of inflammations and allergies of the intestine due to abnormal immunity of the intestinal tract is supported by analyses on the function and differentiation of T cells of patients as well as the cytokine production pattern in lesions or serum. Furthermore, analysis of various recently developed animal models of inflammatory intestinal diseases also reveal that abnormal mucosal immunity causes chronic inflammation in the intestine (Gastroenterology, Vol.109, p.1344-1367, 1995).
For example, it is clear that T cells are deeply involved in the onset of chronic enteritis since inflammation of the intestine develops spontaneously in T cell receptor (TCR) α-chain knockout mice (TCRα−/−) (Cell, Vol.75, p.275-282, 1993; J. Exp. Med., Vol.183, p.847-856, 1996). In colitis of these TCRα−/− mice, the production of IFN-γ in the intestine is elevated, and in the initial stage of inflammation, a rise in IL-1α and IL-1β levels is seen (Laboratory Investigation, Vol.76, p.385-397, 1997). Furthermore, TCRβ (βdim) T cells that have a specific Vβ subset and produce IL-4 can be seen in the digestive tract and lymph nodes (Gastroenterology, Vol.112, p.1876-1886, 1997). In this model, it is thought that a deficiency of TCRαβ T cells causes an increase in the fraction of abnormal T cells, which then causes abnormal regulation of cytokine production, becoming a mediator of inflammation.
In a model in which CD4+/CD45RBhigh T cells are introduced to severe combined immunodeficient mice (SCID mice), severe enteritis accompanying hyperplasia of a mucosal layer and infiltration of lymphocytes in the intestine are induced. However, this enteritis does not occur when unfractionated CD4+T cells are simultaneously introduced (J. Exp. Med., Vol.178, p.237-244, 1993; Int. Immunol., Vol.5, p.1461-1471, 1993). CD4+T cells of SCID mice that have developed enteritis produce IFN-γ. On the other hand, since enteritis is suppressed by the administration of antibodies against INF-γ, Th1 type T cells are considered to cause the inflammation (Immunity, Vol. 1, p.553-562, 1994).
Based on these facts, there seems to be no doubt that CD4+T cells of the intestine and excessive activation thereof are important factors in inflammatory bowel diseases.
Furthermore, regression of enteritis with the decrease in CD4+ T cells in patients affected by both an inflammatory bowel disease and HIV, also supports the deep involvement of abnormal CD4+ T cells in inflammatory bowel diseases (J. Clin. Gastroenterology, Vol.23, p.24-28, 1996). Based on this finding, there have been attempts to treat inflammatory bowel diseases using an anti-CD4 antibody, and it has been reported that inflammatory lesions are suppressed by the administration of an anti-CD4 antibody (Gut, Vol.40, p.320-327, 1997).
On the other hand, such abnormal functional regulation of T cells means that the balance of regulatory cytokine production has collapsed.
In fact, enteritis is also reported to develop spontaneously in IL-2 knockout mice and IL-10 knockout mice (Cell, Vol.75, p.235-261, 1993; Cell, Vol.75, p.263-274, 1993). Furthermore, in these models, excess production of IFN-γ is also observed, supporting the fact that an excessive Th1 type T cell reaction has occurred. Overproduction of IFN-γ in these models is consistent with the observation of increased expression of IFN-γ in lesions seen in Crohn's disease. Enteritis can be treated in IL-10 deficient mice by administering IL-10. It has been reported that enteritis can be suppressed by this method in SCID mice to which CD4+/CD45RBhighT cells have been introduced (Immunity, Vol.1, p.553-562, 1994).
As mentioned above, the analysis of the mechanism of onset of inflammatory bowel diseases has progressed from the aspect of abnormal gastrointestinal mucosal immunity, suggesting the possibility of treating inflammatory bowel diseases by suppressing increased activation of CD4+ T cells and overproduced cytokines. However, the real pathogenesis of inflammatory bowel diseases has not yet been revealed, and furthermore, an effective therapeutic method has not been provided.
The activation of T cells (acquisition of antigen specificity) is initiated when T cells recognize antigens presented by antigen-presenting cells (APCs) such as macrophages, B cells, or dendritic cells. APCs process the incorporated antigens, and the processed antigens are bound to the major histocompatibility antigen complex (MHC) and presented. T cells receive the first signal for cell activation (acquisition of specificity) as a result of the recognition of the processed antigen presented by APCs through a complex formed between the T cell receptor (TCR) on the T cell membrane surface and the antigen (TCR/CD3 complex).
For sufficient activation of T cells, a second signal called the costimulatory signal is necessary in addition to the first signal. T cells are activated antigen-specifically by receiving this costimulatory signal after receiving the first signal.
For this second signal transduction, the interaction (more specifically, the intercellular adhesion mediated by bonds formed between the following molecules) among CD28 (also known as Tp44, T44, or 9.3 antigen), which is a cell surface molecule expressed mainly in T cells and thymus cells, CD80 (also known as B7-1, B7, BB1, or B7/BB1), which is a cell surface molecule expressed by antigen-presenting cells (macrophages, monocytes, dendritic cells, etc.), and CD86 (also known as B7-2 or B70), which is also a cell surface molecule on antigen-presenting cells, is extremely important.
Furthermore, it has been experimentally revealed that the interaction (specifically, the intercellular adhesion mediated by bonds formed between the following molecules) among Cγtolytic T Lymphocyte-associated Antigen 4 (CTLA-4) whose expression is enhanced depending on the second signal, CD80 (B7-1), and CD86 (B7-2) also has an important role in the regulation of T cell activation by this second signal. More specifically, the regulation of T cell activation by this second signal transduction has been revealed to include at least the interaction between CD28 and CD80/CD86, enhancement of the expression of CTLA-4 considered to be dependent on this interaction, and the interaction between CTLA-4 and CD80/CD86.
In addition, recently, similarly to CTLA4 and CD28 described above, a molecule called activation inducible lymphocyte immunomodulatory molecule (AILIM; human, mouse, and rat; Int. Immunol., 12(1), p.51-55, 2000; also called Inducible co-stimulator (ICOS; human; Nature, 397(6716), p.263-266, 1999); J. Immunol., 166(1), p.1,2001; J. Immunol., 165(9), p.5035,2000; Biochem. Biophys. Res. Commun., 276(1), p.335, 2000; Immunity, 13(1), p.95, 2000; J. Exp. Med., 192(1), p.53, 2000; Eur. J. Immunol., 30(4), p.1040, 2000) was identified as the third costimulatory transmission molecule that transduces a second signal (costimulatory signal) necessary for the activation of lymphocytes such as T cells, and coupled with the signal, regulates the function of activated lymphocytes such as activated T cells.
Furthermore, a novel molecule called B7h, B7RP-1, GL50, or LICOS which is considered to be a ligand interacting with the costimulatory transmission molecule AILIM has been identified (Nature. Vol.402, No.6763, pp.827-832, 1999; Nature Medicine, Vol.5, No.12, pp.1365-1369, 1999; J. Immunology, Vol.164, pp.1653-1657, 2000; Curr. Biol., Vol.10, No.6, pp.333-336, 2000).
Exhaustive studies are in progress on the biological functions of these two novel molecules, the functional control of lymphocytes such as T cells through the third costimulatory signal transduction by the molecules.
On the other hand, there has not been even suggestions on the relationship between AILIM (ICOS), which is the third costimulatory transduction molecule considered essential for the activation of T cells such as CD4+ T cells, and the onset of the above-mentioned abnormal immunity of the intestinal mucous membrane and inflammatory bowel diseases (Crohn's disease and colitis (ulcerative colitis and such)). Neither has there been any suggestion on attempts to treat inflammatory bowel diseases by regulating the function of this AILIM molecule.